As an exercise I thought it would be interesting to take html5rocks (an existing HTML5 site) and augment it with a mobile-friendly version. I was mainly concerned with the minimum amount of work required to target smart phones. The goal of my exercise was not to create an entirely new mobile site and maintain two codebases. That would have taken forever and have been a huge waste of time. We had already defined the site's structure (markup). We had a look and feel (CSS). The core functionality (JS) was there. Point is, many sites are in this same boat.